handbook of modern sensors

5
Interface Electronic Circuits
5.1 Input Characteristics of Interface Circuits
A system designer is rarely able to connect a sensor directly to processing, monitoring,
or recording instruments, unless a sensor has a built-in electronic circuit
with an appropriate output format. When a sensor generates an electric signal,
that signal often is either too weak or too noisy, or it contains undesirable components.
In addition, the sensor output may be not compatible with the input requirements
of a data acquisition system, that is, it may have a wrong format. To
mate a sensor and a processing device, they either must share a “common value”
or some kind of a “mating” device is required in between. In other words, the
signal from a sensor usually has to be conditioned before it is fed into a processing
device (a load). Such a load usually requires either voltage or current as its
input signal. An interface or a signal conditioning circuit has a specific purpose:
to bring the signal from the sensor up to the format which is compatible with
the load device. Figure 5.1 shows a stimulus that acts on a sensor which is connected
to a load through an interface circuit. To do its job effectively, an interface
circuit must be a faithful slave of two masters: the sensor and the load device.
Its input characteristics must be matched to the output characteristics of the
sensor and its output must be interfaceable with the load. This book, however, focuses
on the sensors; therefore, we will discuss only the front stages of the interface
circuits.
The input part of an interface circuit may be specified through several standard
numbers. These numbers are useful for calculating how accurately the circuit can
process the sensor’s signal and what the circuit’s contribution to a total error budget is.
The input impedance shows by how much the circuit loads the sensor. The
impedance may be expressed in a complex form as
Z = V I , (5.1)
where V and I are complex notations for the voltage and the current across the input
impedance. For example, if the input of a circuit is modeled as a parallel connection of